This invention relates to a position detecting method and a system therefor, and to a device manufacturing method using the same. More specifically, the present invention is suitable for use in a proximity type exposure apparatus or a stepper type or scan type exposure apparatus, for example, for the manufacture of devices such as semiconductor devices (e.g., IC or LSI), image pickup devices (e.g., CCD), display devices (e.g., liquid crystal panel), or magnetic heads, for example, wherein, when a fine electronic pattern formed on a first object such as a mask or a reticle (hereinafter xe2x80x9cmaskxe2x80x9d) is to be transferred by exposure to the surface of a second object such as a wafer, the spacing between the mask and the wafer is to be measured and is to be controlled to a predetermined value, for relative positioning (alignment) of them.
Conventionally, in exposure apparatus for semiconductor device manufacture, relative positioning of a mask and a wafer is an important factor for improvement of performance. Particularly, in recent exposure apparatuses, very high alignment precision of an order of a submicron or smaller is required because of further enlargement of integration of a semiconductor chip.
In many alignment systems, the spacing between a mask and a wafer is measured by using a surface interval measuring device and, after it is controlled into a predetermined spacing, the alignment operation for the mask and the wafer is performed on the basis of positional information which can be obtained through alignment patterns (alignment marks) provided on scribe lines on the mask and wafer surfaces.
As for an alignment method therefor, U.S. Pat. No. 4,037,969 or Japanese Laid-Open Patent Application, Laid-Open No. 157033/1981 shows the use of a zone plate as an alignment pattern, wherein light is projected to the zone plate and wherein the position of a light convergence point, on a predetermined plane, of the light from the zone plate is detected.
U.S. Pat. No. 4,311,389 shows an alignment pattern provided on a mask surface and having a diffraction optical function like a cylindrical lens, and a dot-like alignment pattern is provided on a wafer surface which functions so that the light quantity of diffraction light of a predetermined order becomes highest when the mask and the wafer are in alignment with each other. By detecting the light from the alignment marks of the mask and the wafer, a relative positional relation between the mask and the wafer is detected.
Among theses detecting methods, a method in which a straight diffraction grating or a zone plate is used as an alignment mark has an advantage that, because the detection is less affected by a fault within the mark, it assures relatively high precision alignment regardless of a semiconductor process.
FIG. 1A is a schematic view of a conventional position detecting system using a zone plate. In FIG. 1A, parallel light is emitted from a light source 72, and it passes through a half mirror 74. After this, the light is collected by a condenser lens 76 to a light convergence point 78. Then, the light is projected on a mask alignment pattern 168a, provided on a mask 168, and a wafer alignment mark 160a, provided on a wafer 160 which is placed on a support table 162.
Each of these alignment patterns 168a and 160a comprises a reflection type zone plate which functions to define a light convergence point upon a plane perpendicular to the optical axis, containing the convergence point 78. The amount of deviation of the light convergence point upon the plane is relayed by a condenser lens 76 and a lens 80 onto a detector 82, whereby it is detected. Then, on the basis of an output signal from the detector 82, a control circuit 84 actuates a driving circuit 164 so that the mask 168 and the wafer 160 are positioned relative to each other.
FIG. 1B is a schematic view for explaining the imaging relation of lights from the mask alignment pattern 168a and the wafer alignment pattern 160a, shown in FIG. 1A.
In FIG. 1B, a portion of light divergently emitted from the convergence point 78 is diffracted by the mask alignment pattern 168a, whereby a light convergence point 78a representing the mask position is formed in the neighborhood of the convergence point 78. Also, another portion of the light passes through the mask 168 as zero-th order transmissive light and, without changing its wavefront, it impinges on the alignment pattern 160a on the wafer 160. Here, the light is diffracted by the wafer alignment pattern 160a and, thereafter, it passes again through the mask 168 as zero-th order transmissive light. Then, it is collected in the neighborhood of the convergence point 78, and it defines a light convergence point 78b which represents the wafer position.
In FIG. 1B, when the light diffracted by the wafer 160 defines a light convergence point, the mask 168 functions as a mere transparent or translucent material. The position of the light convergence point 78b thus defined by the wafer alignment pattern 160a, bears a deviation xcex94"sgr"xe2x80x2 along a plane perpendicular to the optical axis, containing the convergence point 78, the deviation corresponding to a deviation xcex94"sgr" of the wafer 160 with respect to the mask 168, in a direction (lateral direction) along the mask and wafer surfaces.
The deviation xcex94"sgr"xe2x80x2 is then measured with reference to an absolute coordinate system defined on the sensor, whereby the deviation xcex94"sgr" is detected
In the position detecting system shown in FIG. 1A, the intensity of diffraction light from the alignment pattern varies largely due to non-uniform film thickness of a mask membrane, to process dependency of the wafer, or to a difference in film thickness of a resist applied to the wafer surface. Depending on the wafer, therefor, a good signal-to-noise ratio (S/N ratio) for a satisfactory measurement signal light is not obtainable, and alignment signal stability is lowered. This obstructs high precision position detection. As an attempt to this problem, conventionally, light of a different wavelength is used to assure an increased S/N ratio.
However, in conventional examples wherein a zone plate is used as an alignment mark, due to chromatic aberration of the zone plate, diffraction light may not be imaged on a photodetector, causing defocus.
On that occasion, if the diffraction efficiency within the zone plate is not uniform, the detection precision is lowered extraordinarily. Further, if a zone plate corresponding to the different wavelength is provided on a mask, it causes enlargement of the area upon the mask to be occupied by the alignment mark. In a latest semiconductor manufacturing process wherein superposition of not less than 20 (twenty) layers is required, practically, it is difficult to place an alignment mark upon the mask surface.
It is an object of the present invention to provide a position detecting method and/or system by which relative positional deviation between first and second objects can be detected very precisely regardless of a change in state of the first and/or second object, such that high precision alignment is assured.
It is another object of the present invention to provide a device manufacturing method which is based on the position detecting method or system described above.